Deep drawing non-aging cold rolled steel sheet and a method of producing the same



NOW 1965 HIROSHI YOSHIDA 3,215, 7

DEEP DRAWING NON-AGING COLD ROLLED STEEL SHEET AND A METHOD OF PRODUCING THE SAME Filed April 30, 1962 United States Patent O 3,215,567 DEEP DRAWING NON-AGING COLD ROLLED STEEL SHEET AND A METHOD OF PRO- DUCING THE SAME Hiroshi Yoshida, Nobuto-cho, Chiba-shi, Eapan, assignor to Kawasaki Steel Corporation, Kobe-shi, Japan, a corporation of Japan Filed Apr. 30, 1962, Ser. No. 191,369 Claims priority, application Japan, Sept. 16, 1961, 36/ 33,627 4 Claims. (til. 148-31) This invention relates to improvements in a deep drawable non-aging cold rolled steel sheet and a method of producing the same.

Due to the fact that the cold rolled steel sheet is suitable to efliect such a cold work as deep drawing, the material has a broad usage. Accordingly, importance is attached to the forming property thereof, and it has been earnestly demanded by fabricators thereof to make available a more deeply drawable steel sheet at a lower cost.

Heretofore, deep drawing non-aging steel sheet has been produced from aluminum killed steel. In this case, it has been intended to improve the drawability of the product by virtue of an elongated grain structure endowed by the AlN precipitating phenomenon and of the plastic-anisotropy, and also intended to prevent the aging effect by fixing nitrogen as'nitride by aluminum, which nitrogen has been contained in the material.

Currently deep drawing steel sheet is required everywhere and the aluminum killed steel sheet is generally utilized for parts in which stretcher strains and wrinkles are apt to occur. a deep drawing steel sheet has not yet been resolved in view of the technical and economical standpoints. At first, it is substantially impossible to obtain the higher ingot-to-slab yield than in the case of rimmed steel. Secondarily, the final sheet yield is lowered because of the so-called alumina streak which is derived from deoxidation products, aluminum oxide (A1 which appearson the surface of the produced cold rolled steel sheet. Thus, in view of fineness of the steel sheet sur- However, until now the provision of faces, aluminum killed steel sheet is substantially inferior to rimmed steel. Thirdly, in order to obtain the elongated grain-structure or deep drawability, strict quality control is necessary in steel making, where it is difiicult to control a proper content of aluminum, in rolling and annealing. Because of these grounds the aluminum killed steel sheet becomes costlier than rimmed steel sheet. standpoints, it has been earnestly demanded by not only steel makers but also fabricators thereof to produce a deep drawing non-aging steel sheet of rimmed steel which is economically and easily mass producable.

One object of this invention is to answer the demand as above.

By virtue of the method for the production in accordance with this invention, it becomes possible to produce a deep drawing non-aging cold rolled steel sheet. In accordance with this invention, a raw material for steel is so refined that a quantity of phosphorus remains in molten steel during the steel making process or a quan tity of phosphorus is added to molten steel so as to produce an ingot containing more phosphorus than the ingot in accordance with the usual cold rolled steel sheet standard. After slabbing usually, hot-rolling by a usual hot strip mill, pickling, and cold-rolling by a usual cold strip mill, the cold rolled steel sheet is subjected to, for

In view of the above technical and economical- Patented Nov. 2, 1965 example, an open coil annealing in a decarburizing and denitriding atmosphere containing substantially hydrogen. Through such a process, it has been found that it is possible to produce a deep drawing non-aging cold rolled steel sheet.

The most essential feature of this invention is the method for the production characterized in that a cold rolled steel sheet containing more than 0.030 percent and less than 0.120 percent of phosphorus is decarburized and denitrided so as tomake carbon less than 0.02 percent or less than 0.015% and nitrogen less than 0.0025 percent or less than 0.0010%. Heretofore, it was believed that it was detrimental to contain such impurities as carbon, phosphorus, sulphur and the others in steel for a deep drawing steel sheet. In general, upper limits of carbon, phosphorus, sulphur and other impurities are indicated in various standards of the cold rolled steel sheet. For example, referring to the Japanese Industrial Standard, the standard of a deep drawing cold rolled steel sheet SPC-3 indicates less than 0.08 percent of carbon, less than 0.030 percent of phosphorus, and less than 0.040 percent of sulphur, while the standard of a commercial grade cold rolled steel sheet SPC-l indicates less than 0.12 percent of carbon, less than 0.045 percent of phosphorus, and less than 0.050 percent of sulphur. In accordance with the Deutsche IndustrieNormal, an extra'deep drawing cold rolled steel strip ST-4 indicates less than 0.10 percent of carbon, less than 0.030 percent of phosphorus, and less than 0.035 percent of sulphur, while a deep drawing cold rolled steel strip ST-2 indicates less than 0.10 percent of carbon, less than 0.060 percent of phosphorus, and less than 0.050 percent of sulphur. Practically speaking, the steel maker would especially mind reduction of impurities in steel by being particular about raw materials and paying attention to the refining in case such a high grade steel sheet as an extra-deep drawing cold rolled steel sheet is produced. In view of the above facts, it is apparent that it has been commonly understood that carbon, phosphorus, sulphur and other impurities are generally detrimental to a deep drawing steel sheet. Notwithstanding, I have found that, in a steel sheet of extra-low carbon content, a quantity of phosphorus not only causes a steel sheet to prevent the so-called cold shortness particularly but brings an unusually advantageous result for the deep drawability. Thus I have found to produce a high phosphorus containing extra-deep drawing steel sheet contrary to the expectations of those skilled in the art.

Another object of this invention is to provide inexpensive rimmed steel, capped steel, and semi-killed steel which are adapted to produce deep drawing non-aging cold rolled steel sheet, the drawability being more excellent than that of the usual aluminum killed extradeep drawing steel sheet.

Further another object of this invention is to provide a method of producing a deep drawing steel sheet'more economically.

The invention will be better understood and other objects and additional advantages of the invention will become apparent upon perusal of the following description taken in connection with the drawings, in which:

FIG. 1 is a cross-sectional diagram for explaining the conical cu-p testing apparatus; and

FIG. 2 shows samples of the cold rolled steel sheets in accordance with this invention as well as commercial grade rimmed steel and deep drawing aluminum killed steel sheet, in which the draw through and the fractures appeared after conical cup tests have been subjected thereto, respectively.

Many steel sheets were studied by me with respect to drawability and other mechanical properties, varying with phosphorus content, carbon and nitrogen contents, deoxidizing method, ingot casting method, quantities of other addition elements such as silicon, aluminum, and the like.

As a testing method for drawability, the conical cup test was adopted. In accordance with this method, a disc blank is pressed into a conical die having a vertical angle of 60 as shown in FIG. 1, until a fracture of the blank appears. The conical cup value which may be abbreviated C.C.V. is determined by a mean value of the maximum and minimum of the flange diameters of the blank pressed into the die as above when the fracture has appeared.

Now the deep drawing non-aging cold rolled steel sheets embodying this invention will be explained.

Table I shows drawability and other mechanical properties of various cold rolled steel sheets, some of such conditions as phosphorus content, carbon content, nitrogen content, deoxidizing method, ingot casting method, silicon content, aluminum content, or the like being different with each other. The conical cup values shown in Table I are results of the following test conditions:

Further as a test for aging, the yield elongation after aging for a period of one month was measured, whereby occurrence of the stretcher-strain is judged.

In the Table I, No. 1 represents a sample of well known commercial grade rimmed steel sheets, Nos. 2 through 5 represent samples of the steel sheet shown as No. 1, decarburized and denitrided down to various contents, respectively, Nos. 6 through 16 represent samples of rimmed steel sheets, phosphorus contents thereof being various, and being decarburized and denitrided down to various contents, respectively, in which samples of Nos. 9 through 15 fall under the scope of this invention, No. 17 represents a sample of well known aluminum killed deep drawing steel sheet, No. 18 represents a sample of aluminumkilled deep drawing steel sheet containing high phosphorus, No. 19 represents a sample of mechanical capped steel sheet containing high phosphorus, No. 20 represents a sample of the steel sheet of No. 19 decarburized and denitrided, which falls under the scope of this invention, No. 21 represents-a sample of chemical capped steel sheet containing high phosphorus, No. 22 represents a sample of the steel sheet of No. 21 decarburized and denitrided, which falls under the scope of this invention, No. 23 represents a sample of semi-killed steel sheet containing high phosphorus, and No. 24 represents a sample of the steel sheet of No. 23 decarburized and denitrided, which falls under the scope of this invention.

FIG. 2 shows seven disc blanks subjected to such a conical cup test as explained hereinbefore. The seven disc blanks arranged from left to right in the view correspond to the samples Nos. 6, 1, 5, 17, 10, 13, and 11 in Table I, respectively.

other steel sheets [Norm-The underlined numbers represent samples of steel in accordance with this invention] Kind of Steel Sheet Comm. Grade Comm. Grade Comm. Grade Comm. Grade Comm. Grade P cont. Rimmed Rimmed Steel Rimmed Steel Rimmed Steel Rimmed Steel Rimmed Steel Steel Sheet. Sheet. Sheet. S eet. Sheet. Sheet. Kind of Annealing Usual Bell An- De-C DeN An- De-C DeN .An- De-C DeN An- De-C DeN An- Usual Bell Annealing. nealing. nealing. nealing. nealing. 5 0.023 0.018 0.011

N Conical Cup Value Yield kgJPoint, mm. 25.3.-- 23.0.-- 20.1 19.9 18.0 27.8. Tensile Strength, 33.2 29.5 28.7 27.4.- 26.8-- 38.9.

kgJmmfl. Elongation, percent 44 47 48 51 52 38. Yield Elongation after 3.6. 2.7 2.3--- 2.0 1.0 3.0.

Aging for 1 mth., percent.

Number- 7 8 2 19 E 1 1' Kind of Steel Sheet P cont. P cont. P cont. P cont. P cont. P cont. P cont.

Rimmed Rimmed Rimmed Rimmed Rimmed Rimmed Rimmed Steel Sheet Steel Sheet. Steel Sheet. Steel Sheet Steel Sheet Steel Sheet Steel Sheet. Kind of Annealing De-C De-N De-C De-N De-C De-N De-C De-N De-C De-N De C De-N De-C De-N Annealing Annealing.

.01 0.00s. SL Trace Trace. Mn 0.36- 0.27. P- 0.066. 0.065. S 0.016- 0.019. AL- Trace Trace. N- 0.0015 0.0010. Conical Cup Value Draw Thro..- 33.12. Yield Point, kg./mm. 20. Tensile Strength, kgJmm. 33.6. 33.5. Elongation, percent 44 45. Yield Elongation after Aging for 0.2- 0.0.

1 mth., percent.

Table IContinued Number 1 4 1 5 16 17 18 Kind of Steel Sheet P cont. Rimmed P cont. Rimmed P cont. Rimmed Al-Killed Deep P cont. Al-killed Steel Sheet. Steel Sheet. Steel Sheet. Drawing Steel Deep Drawing Sheet. Steel Sheet. Kind of Annealing De-O De-N Anneal- De-C De-N Anneal- De-C De-N Anneal- Usual Bell Annealing. Usual Bell Annealing.

mg. mg. mg. C 0.012- 0.014- 0.011 0.035 0.044. Si-.- Trace. Trace.. Trace-. Trace Trace. Mn 0.36. 0.33- 0.29. 0.33. 0.26. P 0.050 0.046 0.020 0.008- 0.065. S 0.014- 0.028. 0.015. 0.015- 0.015. Al Traee n Trace Trace 0.040 0.042. N-. 0.0010--- 0.0012. 0.0013. 0.0060. 0.0049. Conical Cup Value- 34.75. 36.70-- 37.23. 36.88- 37.20. Yield Point, kg./mm. 19.8. 19.9. 18.5. Tensile; Strength, kg./ 32.4. 31.2- 28.4- 30.5. 34.4.

mm. Elongation, percent- 46 48 48 46 42. Yield Elongation after 0.2 0.5- 1.2- 0.0 0.0.

Aging for 1 mth., per cent.

Number 19 22 21 22 23 24 Kind of Steel Sheet P cont. Mech. P cont. Mech. P cont. Chem. P cont. Chem. P cont. Semi- P cont. Semi- Capped Steel Capped Steel Capped Steel Capped Steel Killed Steel Killed Steel Sheet. Sh Sheet. Sheet. Sheet. Sheet Kind of Annealing Usual Bell De-C De-N Annealing. Annealing Si Trace. 0.05.

MIL-.. 0.34. 0.33.

A1- Trace. Trace Conical Cup Value 38.88 Draw Thro.

Yield Point, kg./mm. 26.2

Tensile Strength, kg./ 37.4. 34.3.

mm. Elongation, percent 41 44 44. Yield Elongation after 3.0 0.0 0.0.

Aging for 1 mth., percent.

As admittedly shown in Table I and FIG. 2, steel sheet made merely to contain phosphorus without decarburization and denitridation shows somewhat lower drawability.

In case where the phosphorus content is in a low range and a decarburizing and denitriding annealing is subjected, the drawability is improved in accordance with removal of carbon which is a detrimental component, and removal of some quantity of nitrogen. However, the effect is not substantially large. Even if the decarburization and denitridation have been efiected, it is impossible that a rimmed steel surpasses aluminum killed extra-deep drawing steel sheet usually processed.

In addition, in such a case as above, the tensile strength would be lowered remarkably, and it would be lowered, in some cases, down to less than 28 kg./mm. of the deep drawing steel sheet SPC-3 indicated in the Japanese Industrial Standard. It is considered that such a shortage of tensile strength as above is not only a problem on the specification but also a problem to be considered in the practical use.

In case where, keeping the phosphorus content in a range of 0.030 to 0.120 percent, carbon content has been reduced down to less than 0.02 percent and nitrogen content has been reduced down to less than 0.0025 percent by decarburization and denitridation annealing, the drawability of :such an annealed sheet has been surely improved, comparing with a case of a less phosphorus content. Moreover, it would be deemed that the former has been prevented from the lowering down of tensile strength sufiiciently.

Thus the more the carbon and nitrogen contents are lowered by decarburization and denitridation, the more the yield elongation after aging treatment is reduced, whereby the sheet becomes to approach non-aging property.

It is deemed in Table I that, a high phosphorus containing steel has a remarkably less yield elongation after aging treatment comparing with the usual rimmed steel. In other words, the high phosphorus containing steel is more advantageous for obtaining a non-aging steel sheet.

In fact if the quantities of phosphorus, carbon and nitrogen are within the above range, a yield elongation after aging treatment is kept within a range to be called nonaging property.

However, in case the phosphorus content is lower than the lower limit of the above mentioned range, the effect is reduced, and in case the phosphorus content surpasses the upper limit thereof, the drawability becomes inferior contrariwise. It is to be noted that at a phosphorus content of 0.070 percent or thereabout it is possible to obtain a more excellent result by inexpensive rimmed steel, capped steel, or semi-killed steel than by costly aluminum killed deep drawing steel sheet.

Although the reason why the above stated effect is exerted by phosphorus has not been particularly clear, it is considered that, while phosphorus affects the draw abi lity considerably disadvantageously in case it co-exists with such interstitial atoms as carbon, nitrogen and the like, the phosphorus is likely to increase strain hardening property, prevent the material from a descent of tensile strength, and improve dra'wability contrariwi'se in case carbon and nitrogen existing in the free state are reduced extremely.

The followng example illustrates a method of carrying this invention into effect. However, this description will be understood to be illustrative of the invention and not as limiting it to the particular method described.

Usual scrap iron and pig iron were molten in a usual basic open heart-h furnace having a capacity of tons and refined as in the case of usual low carbon rimmed steel. The steel bath was chemically analyzed just before tapping as follows:

C Si M11 P S 0. 06% Trace 0. 07%

Si Mn P s 0. 08% Trace o. 35% o. 050% 0. 015% The molten steel was cast into open top ingot molds, 1380 x 600 X 2300 mm., yielding 10 tons of rimmed ingots. The ing-o-t was soaked in a soaking pit at a temperature of about 1300 C. Subsequently, the ingot was rolled down to a 160 X 1270 x 6 100 mm.-sized slab weighing 9310 kg. by a usual slabbing mill. Then the slab was hot-rolled down to a coil of 2.8 mm. in thickness and 1243 mm. in width, weighing 9030 kg. by a usual hot strip mill. The hot rolling was started at a temperature of about 1200 C. and finished at a temperature of about 880 C. After a usual pickling, the hot rolled coil was cold-rolled down to a coil of 0.8 mm. in thickness and 1240 mm. in width, weighing 8760 kg. by a usual cold strip mill. The cold rolled coil was then decarburized and denitrided. The decarburization and denitridation was done employing an open coil annealing furnace by using of AX-gas, the soaking temperature being about 720 C., and the soaking time being 40 hours. At last a skin pass rolling, reduction rate being 0.5 percent, was subjected to the steel sheet as usual, yielding a steel sheet illustrated as No. ,14 in Table I.

8 Although I have shown and described certain particular embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art or by the spirit of the appended claims.

What I claim is: v 1. A deep drawable non-aging cold rolled steel sheet I characterized by having a good drawability represented 'by a conical cup value determined by the conical cup test, said value being less than about 36.70 for a sheet of the thickness of 0.8 mm., said steel consisting essentially of less than .020% carbon, 0.0300.120% phosphorus, less than 0.0025 nitrogen and the remainder iron and incidental impurities, said steel sheet being produced by subjecting a cold rolled steel sheet containing 0.030 0.120% phosphorus to'a decarburizing and denitriding anneal. v

2. The steel sheet according to claim 1 wherein the carbon content is less than 0.015%.

3. The steel sheet according to claim 2 wherein the nitrogen content is less than 0.0010%.

4. The steel sheet according to claim 1 wherein the cold rolled sheet is of rimmed steel.

References Cited by the Examiner OTHER REFERENCES Archiv fur Das Eisenhuttenwesen, vol. 8, 1934-35, pages 26367.

ASM Metals Handbook (1948 edition), page 234 relied on The Making, Shaping, and Treating of Steel, by US. Steel Corp. (7th edition), page 364 relied on.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner. 

1. A DEEP DRAWABLE NON-AGING COLD ROLLED STEEL SHEET CHARACTERIZED BY HAVING A GOOD DRAWABILITY REPRESENTED BY A CONICAL CUP VALUE DETERMINED BY THE CONICAL CUP TEST, SAID VALUE BEING LESS THAN ABOUT 36.70 FOR A SHEET OF THE THICKNESS OF 0.8 MM., SAID STEEL CONSISTING ESSENTIALLY OF LESS THAN .020% CARBON, 0.030-0.120% PHOSPHORUS, LESS THAN 0.0025% NITROGEN AND THE REMAINDER IRON AND INCIDENTAL IMPURITIES, SAID STEEL SHEET BEING PRODUCED BY SUBJECTING A COLD ROLLED STEEL SHEET CONTAINING 0.0300.120% PHOSPHORUS TO A DECARBURIZING AND DENITRIDING ANNEAL. 